1. Field of the Invention
This invention relates to steel sheets for painting, which are used by the forming such as press forming or the like as an outer panel for automobile or a decorative outer plate for electric appliances, and a method of producing the same. More particularly, it relates to steel sheets for painting having an improved distinctness of image after painting and a method of producing the same.
Here, the term "steel sheet" used herein means to include cold rolled steel sheets, surface-treated steel sheets, hot rolled steel sheets and so on, which are capable of being subjected to a painting treatment.
2. Related Art Statement
In general, the formable thin steel sheets such as cold rolled thin steel sheets are produced by subjecting the steel sheet after the cold rolling to degreasing, annealing and temper rolling in this order. In this case, the temper rolling is to improve the galling resistance in the press forming by conducting a light rolling through work rolls having a dulled surface to give a proper surface roughness to the steel sheet surface.
As a process for dulling the surface of the work roll to be used in the temper rolling, there have hitherto been practised a shot blast process and a discharge working process.
When the work roll is subjected to a dulling according to these processes, an irregular section profile is formed on the surface of the work roll, and consequently the steel sheet after the temper rolling using such a work roll indicates a rough surface comprising a plurality of irregular mountain and valley portions. If such a surface roughened steel sheet is subjected to a press forming, a lubricating oil is reserved in the valley portions to reduce friction force between press mold and steel sheet and hence make the press operation easy, while metallic debris exfoliated by the friction force to the mold are trapped in the valley portions to prevent the galling.
Recently, the finish feeling after painting on vehicle body in passenger cars and trucks is a very important quality control item because the height in synthetic quality of automobile can directly be appealed to the eye of the user as a good finish quality.
Now, there are several evaluation items on the painted surface. Among them, it is particularly important that a glossiness lessening irregular reflection on the painting surface and an image clarity defining few image distortion are excellent. In general, the combination of the glossiness and the image clarity is called as a distinctness of image.
The distinctness of image on the painted surface is dependent upon the kind of paint and the painting process, and is also strongly influenced by the rough surface of the steel sheet as a substrate. That is, when the unevenness of the steel sheet surface is much, the painted surface becomes much uneven, and consequently the irregular reflection of light is caused to damage the glossiness and also the image distortion is produced to deteriorate the image clarity, so that the distinctness of image is degraded.
Generally, the section profile of the steel sheet surface is divided into a roughness curve and a waviness curve. Heretofore, the distinctness of image in the painted surface has been determined by a centerline average roughness (Ra) in the roughness curve. In this case, it is known that as the value of Ra becomes larger, the amplitude between mountain portion and valley portion is large and hence the unevenness of the painted surface becomes large and consequently the distinctness of image is degraded. On the other hand, the waviness curve has not been noticed as a method for evaluating the distinctness of image at all.
As the method for evaluating the distinctness of image after painting, there have been developed various systems. Among them, a value measured by means of a Dorigon meter made by Hunter Associates Laboratory or a so-called DOI value is most usually used. The DOI value is expressed by the following equation: EQU DOI=100.times.(Rs-R.sub.0.3)/Rs,
wherein Rs is an intensity of a specular reflected light when a light entered at an incident angle of 30.degree. is reflected at a specular reflective angle of 30.degree. with respect to a sample, and R.sub.0.3 is an intensity of a scattered light at a reflective angle of 30.degree..+-.0.3.degree..
Further, an image clarity (C, %) measured by means of an image measuring meter (HA-ICM model) made by Suga Shikenki K.K. is also usually used. In this case, a quantity of light reflected on a sample is measured through a moving optical comb, from which is calculated an image clarity or image definition (C, %) indicating a combination of image clearness, image distortion and haze in the visual feeling process.
The optical comb is made so as to match with a chart scale. In the measurement, a parallel light passing from a light source through a slit having a width of 0.03.+-.0.005 mm is reflected on a sample. The reflected light is focused through a lens and received on a light receiving means through an optical comb moving in left and right directions. The change of light quantity detected by the light receiving means is converted into a wave form through an instrument device connected to the light receiving means, from which the image definition (C, %) can be calculated.
Here, the image definition (C, %) is defined by the following equation: ##EQU1## wherein M is a maximum value of light quantity transmitted from a transparent portion of the optical comb and m is a minimum value of light quantity transmitted from an opaque portion thereof. The larger the C value, the higher the image clarity, while the smaller the C value, the larger the amount of haze or image distortion.
When the steel sheet is subjected to a temper rolling with work rolls dulled through the conventional shot blast process or discharge working process, it exhibits a rough surface comprised of irregular mountain and valley portions as previously mentioned. If the painting is applied to the steel sheet having such irregular mountain and valley portions, since the coating is formed along the slopes of the mountain and valley portions, the distinctness of image is degraded. That is, such a problem can not be avoided in the steel sheets for painting temper rolled with work rolls through the shot blast process (hereinafter referred to as SB sheet) and discharge working process (hereinafter referred to as ED sheet), so that it is very difficult to provide a sufficiently improved distinctness of image on the painted surface. That is, the dull pattern in the SB and ED sheets is random and the reproducibility thereof is considerably poor, so that the scattering of the distinctness of image after painting becomes large.
On the other hand, when the center-line average roughness Ra in the SB and ED sheets is made too small for improving the distinctness of image, the amount of lubricating oil held in the sheet is reduced in the press forming to cause the galling phenomenon or the like, resulting in the reduction of operation efficiency, deterioration of quality, decrease of yield and the like.
Therefore, the SB and ED sheets can not satisfy the simultaneous establishment of press formability and distinctness of image after painting, so that they can not be adopted as a means for improving the distinctness of image after painting.